Fiber-reinforced resins in which a resin is fiber-reinforced have light weight and high strength and have been used for various sporting goods, construction materials, aircraft, and the like and uses thereof have been extended to special automobile parts and the like, but the application thereof has not been advanced to general automobile parts where economical efficiency and mass productivity are regarded as important. However, with a change in social situations where the reduction of loads on the environment is required, the development of an automobile weight-reducing technology is becoming an important challenge and attention is being attracted to the application of the fiber-reinforced resins to the general automobile parts in which the use of a large amount of the resins is expected.
Of the fiber-reinforced resins, in consideration of mass production, easiness in handling and high productivity, applicability to various purposes, and the like, attention is focused on a fiber-reinforced resin in which a reinforcing fiber material such as a reinforcing fiber bundle or a reinforcing fiber woven fabric is impregnated with a thermoplastic resin. However, since a thermoplastic resin has high viscosity, it is not easy to impregnate the reinforcing fiber material therewith and there are problems that its homogeneous impregnation is difficult or air bubbles remain therein, so that various proposals have been made for solving such problems.
For example, PTL 1 proposes a method for manufacturing a sheet-like prepreg in which a pressure-reduced seal portion, a temperature-controllable connecting portion, and a die containing a molten resin stored therein are connected in tandem and a band-like reinforcing fiber bundle composed of a plurality of continuous reinforcing fibers is inserted from an introduction port at one end of the seal portion through the connection portion and the molten resin in the die and taken out of an end of the die. According to the manufacturing method, it is said that the impregnation with the resin is homogeneous and satisfactory and the impregnation time is short.
PTL 2 proposes a method for manufacturing a fiber-reinforced resin sheet by introducing a sheet-like reinforcing fiber substrate and a thermoplastic resin between a pair of rollers and impregnating the reinforcing fiber substrate with the thermoplastic resin in a molten state while rotating the pair of rolls, wherein a metal-made main roller and a metal-made press roller are used as the pair of rolls and the reinforcing fiber substrate is impregnated with the thermoplastic resin while the circumferential surface of the press roller is deformed so that the circumferential surface of the press roller conforms to the circumferential surface shape of the main roller by pressing the press roller against the main roller. According to the manufacturing method, since not a line pressure but a surface pressure can be applied to the thermoplastic resin for impregnation, it is said that homogeneous and satisfactory impregnation can be performed without uneven pressurization.
PTL 3 proposes a method for manufacturing a fiber-reinforced thermoplastic resin, the method comprising a disposing step of disposing a thermoplastic resin layer composed of a thermoplastic resin on one surface of a reinforcing fiber sheet constituted by a reinforcing fiber composed of a long fiber and disposing a net-like sheet composed of a material which does not melt at a temperature at which the thermoplastic resin melts, on another surface of the reinforcing fiber sheet to obtain a laminate and an impregnating step of heating and pressurizing the laminate at the temperature at which the thermoplastic resin melts but the net-like sheet does not melt to impregnate the reinforcing fiber sheet and the net-like sheet with the thermoplastic resin. According to the manufacturing method, since cavities can be reduced by thickening the thermoplastic resin layer and excessive thermoplastic resin can be transferred to the net-like sheet, it is said that there can be obtained a fiber-reinforced thermoplastic resin which has high fiber content and excellent strength, suppresses fiber meandering due to high resin content, and also has less cavities such as voids and satisfactory impregnation properties.